[mediaqueries] Add example to update-frequency.

Author: tabatkins

mediaqueries/Overview.bs

@@ -1135,10 +1135,24 @@ update-frequency</h3>
 		<dt><dfn>normal</dfn>
 		<dd>
 			The layout may change dynamically according to the usual rules of CSS,
-			and the output device is not unusually constrained in speed.
+			and the output device is not unusually constrained in speed,
+			so regularly-updating things like CSS Animations can be used.
 			Example: computer screens.
 	</dl>
 
+	<div class='example'>
+		For example, if a page styles its links to only add underlines on hover,
+		it may want to always display underlines when printed:
+
+		<pre>
+		a { text-decoration: none; }
+		a:hover, a:focus { text-decoration: underline; }
+		@media (update-frequency: none) {
+			a { text-decoration: underline; }
+		}
+		</pre>
+	</div>
+
 <h3 id='mf-overflow-block'>
 overflow-block</h3>